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Dark force could keep Milky Way’s neighbours away

By Anil Ananthaswamy

DARK energy is thought to be ripping apart the fabric of space-time on cosmological scales. But it now seems it may also have an influence on the scale of a single galaxy. If so, it could explain why the Milky Way has fewer dwarf galaxies orbiting it than expected.

Astronomers came up with dark energy to explain why the expansion of the universe is accelerating. But it has so far only been studied on scales spanning much of the universe.

“Most people think that on shorter distance scales dark energy doesn’t do anything,” says Stephen Hsu of Michigan State University in East Lansing. At short distances, the other forces – including gravity – should be strong enough to counter dark energy’s repulsive influence.

But Hsu and his colleagues wondered how far from the middle of a galaxy you have to go before dark energy takes over. Their calculations show that every galaxy has a critical radius from its centre where the gravitational pull of the galaxy’s mass is balanced by dark energy.

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For massive galaxies like the Milky Way, this critical radius is 10 times the galaxy’s radius, so nothing inside large galaxies would be affected by dark energy. But for dwarf galaxies, the critical radius can be much smaller. (arxiv.org/abs/1501.05952).

This effect could solve a long-standing mystery called the missing satellite problem. Astronomers see far fewer dwarf galaxies orbiting the Milky Way than simulations predict should be there. But according to Hsu’s calculations, dark energy should prevent anything from orbiting the Milky Way beyond its critical radius by deflecting the dwarfs before gravity catches them – explaining why no such satellites have been found.

“It’s possible that the missing satellite problem is just a manifestation of the fact that there is actually a repulsive force involved,” says Hsu.

This article appeared in print under the headline “Dark force keeps galactic dwarfs away”